Chromogenic ionophores (or ionophore dyes) are a class of color-responsive reagents for detecting alkali and alkaline earth metal ions. In these reagents, a size-specific ionophore group (the ion recognizing ionophore) is fused to an aromatic ring which in turn is functionalized with a chromophore such as an azo, or picrylamino group. Chromogenic ionophores have been extensively studied (see H-G. Lohr and F. Vogtle, "Chromo and Fluoroionophores. A New Class of Dye Reagents," Acc. Chem. Res. 1985, 18, 65-72; M. Tagaji; and K. Uneo, "Crown Compounds as Alkali and Alkaline Earth Metal Ion Selective Chromogenic Reagents," Top. Curt. Chem. 1984, 121, 39-65). There are two classes of chromogenic ionophores, those that show a pH-dependent response and those that function at neutral pH. The former class of ionophores shows dramatic changes in color. However, the colored form is usually detected in organic solvents and hence an extraction step is essential in addition to adjustment of the pH of the system. Chromoionophores presently known to the art, that function at neutral pH, have yet to demonstrate a sufficient change in color. Without an easily detectable change in color, these chromoionophores cannot be useful as analytical reagents and the like.
A second type of ionophore is the "fluoroionophore." The measurement of fluorescence quenching or enhanced fluorescence emission when metal ions are bound to these fluorogenic ionophores is more accurate than measurements based on chromogenic phenomena. This is because fluorescence measurements are made against a dark background, whereas chromogenic methods require detection of absorption maxima or changes in absorption coefficients. Among the fluorogenic ionophores reported in the literature are those described by Nishida, et al. "Fluorescent Crown Ether Reagent For Alkali and Alkaline Earth Metal Ions," Chem. Lett., pp. 1853-1854, (1982), by Kenneth W. Street, Jr. and Shelly A. Kraus in "A New Metal Sensitive Fluorescence Reagent," Anal. Lett., 19 (7 and 8), 735-745 (1986), and by A. P. deSilva et al., "Fluorescent Signaling Crown Ethers: `Switching On` of Fluorescence by Alkali Metal Ion Recognition and Binding in situ" J. Chem. Soc., Chem. Commun. 1986, 1709-10. However, all the above ionophores suffer from a disadvantage in that they are pH dependent, and can function only at a pH much higher than that of normal body fluid, and hence cannot be used for in vivo applications.